I have a Retrokits RK-002 for sale. Light use; it just didn't fit my setup and workflow. Comes with the retail info page.
It requires MIDI power from from the MIDI port that the black end is plugged into in order to be programmed and to be used. You can find lists of compatible hardware online. The device I've been using is a standard Roland UM-ONE mk2 USB-MIDI interface, which works great.
This cable, when programmed with one of the many firmwares available on the Retrokits DUY Portal, here:
https://duy.retrokits.com/
...can do whatever MIDI processing that you need to customize some functionality of the connected device. Commonly used to add missing functionality to Korg Volcas and Elekton devices such as the Digitakt, amongst others. If you want it programmed with a specific firmware I'd be happy to preprogram it in advance of selling.
In addition to using one of the many available user firmwares, you can also program it with your own custom firmware. The programming language is basically standard Arduino C, but Retrokits have their own libraries which simplify many of the things one would want to do with incoming or outgoing MIDI through the cable.
Details: cash/local pickup only. Will not ship. Reply with your phone number and we can text to set up a place/time to meet. I can meet near 12 Street or Lake Merritt BART stations. Thanks!